Role of Texture and Microstructural Developments in the Forming Limit Diagrams of Family of Interstitial Free Steels

Forming limit diagrams (FLDs) are widely used in sheet metal industries to assess formability. These are graphical representations of major and minor strains on a 2-D plot separating safe and unsafe regions. Limiting strains were measured by digital image correlation (DIC) technique. In the present work, microstructure evolution and forming behavior of family of interstitial free steels: interstitial free (IF) and interstitial free-high strength (IF-HS) grades have been investigated. Both experimental and finite element (FE) simulated FLDs indicated higher formability for IF steel. Microstructural developments affect forming limits and influence forming limit diagrams. Evolving microstructure during forming was studied by texture developments using x-ray diffraction (XRD) and in grain average misorientation developments using electron backscattered diffraction (EBSD) techniques as a function of strain and strain paths. Estimated microstructural parameters revealed that the enhanced formability of IF steel was due to the presence of strong γ-fiber (ND//<111>) recrystallization texture and corresponding absence of θ-fiber (ND//<100>). On the contrary, IF-HS steel showed the abundant θ-fiber component and hence decreased formability.